An avalanche photodiode (APD) is a semiconductor device in which charge carriers are generated and multiplied when exposed to light. They are widely used in high speed communication. APDs operate under reverse bias with a high peak electric field close to breakdown. Incident photons in the appropriate wavelength range, i.e., 300-1600 nm, create charge carriers (electrons and/or holes) in the semiconductor material. Charge carriers are accelerated toward opposite electrodes by the large reverse bias. The accelerated carriers then produce secondary carriers by impact ionization within the semiconductor material. The resultant avalanche can produce gains in excess of 10.sup.3. APDs can improve the sensitivity of optical receivers by more than 10 dB.
For long wavelength applications, e.g., 1000-1600 nm wavelengths of light, a light absorption layer must be formed from a narrow band-gap semiconductor material. However, the large reverse bias typically creates excessive noise due to a large dark current flowing through the narrow band-gap material. To suppress this excessive noise, a separate layer having a wider band-gap is provided, allowing avalanche multiplication to take place. An APD constructed in this manner is commonly known as a separate absorption and multiplication (SAM) structure.
Generally, there are two types of SAM-APDs: planar structure or mesa structure. In the mesa structure SAM-APDs, the doping and thickness of the multiplication layer is controlled by epitaxial technology, providing precise control over the layer's thickness and impurity dopant concentration. Mesa structures, however, expose a high electric field region at the surface. The passivation of the surface has not been adequately demonstrated to date, and mesa structure APDs are, therefore, not favored.
Properly designed planar APDs exhibit lower electric fields at the surface of the structure than mesa structure APDs, and are commonly formed by diffusing p-type dopants into epitaxially grown n-type layers. The thickness of the multiplication layer is defined by the position of the diffused junction. FIG. 1 shows a prior art planar SAM-APD in which an n InP buffer layer 10, an n.sup.- InGaAs light absorption layer 11, an n.sup.- InGaAsP intermediate layer 12, an n avalanche InP multiplication layer 13, and an n.sup.- InP window layer 14 are epitaxially grown in sequence on an n.sup.+ InP substrate 15. A p.sup.+ InP diffusion layer 16 and a p guard ring 17 are formed in the window layer 14 by selective diffusion or ion implantation techniques. A P-side electrode 18 is provided on the upper surface of the device, and an N-side electrode 19 is formed on the lower surface of the substrate 15.
In the SAM-APD thus formed, holes generated by light absorption in the n.sup.- InGaAs layer 11 drift to the n InP layer 13 to initiate avalanche multiplication. Ideally, the APD is designed so that the field in the InGaAs layer 11 can be kept low enough to suppress the dark current. In the valence band of the hetereojunction formed between the n.sup.- InGaAs layer 11 and the n InP layer 13, holes generated in the n.sup.- InGaAs layer 11 are accumulated. This reduces the response of the APD. To overcome this disadvantage, the n.sup.- InGaAsP intermediate layer 12 is disposed between the n.sup.- InGaAs layer 11 and the n InP layer 13.
To obtain high sensitivity, it is necessary to obtain uniform avalanche multiplication along the P-N junction 21. To that end, it is necessary to restrict the region of breakdown along the central portion of the APD, coextensive to a planar portion of the P-N junction 21. It has long been recognized that electric fields concentrate in the curved portion 20 of the P-N junction 21 between the n.sup.- InP window layer 14 and the p.sup.+ InP layer 16. This field concentration can lead to premature breakdown at the curved portion 20, commonly known as edge breakdown.
To avoid edge breakdown, guard ring 17 is provided to surround the p.sup.+ InP layer 16. The guard ring 17 is formed so that it creates a second P-N junction 22 between both the window layer 14 and the multiplication layer 13. The second P-N junction is generally deeper than the P-N junction 21 to eliminate the curved portion 20.
In the prior art APD, described above, the n.sup.- InP window layer 14 often has a low carrier concentration and is epitaxially grown on the n InP avalanche multiplication layer 13, which has a higher charge carrier concentration. P-N junction 22 is formed by selective diffusion, or implantation and annealing, at high temperatures of Be ions or the like into the window layer 14. P-N junction 21 is typically formed by selective diffusion of the p dopants of layer 16 into layer 14 using Cd or Zn as a diffusion source.
In order to achieve an APD with high gain-bandwidth product, P-N junction 21 is positioned as deep as possible near or within the avalanche multiplication layer 13. Further, to obtain a good response time, it is necessary to obtain a high concentration of dopants in the multiplication layer. This requires a high degree of control of both the doping and thickness of the multiplication layer 13 and the guard ring 17 in order to extract a sufficient quantity of photo-generated carriers to achieve the desired gain. Also, the electric field in the absorption layer must be kept low to avoid excessive dark current.
There have been many prior art attempts to suppress noise and increase gain by taking advantage of the aforementioned principles. U.S. Pat. No. 5,308,995 to Tsuji et al. discloses a superlattice APD having improved gain-bandwidth and ionization rate. The multiplication layer is formed by two or more types of semiconductor layers with differing band gaps. A tensile stress is applied to the barrier layers, i.e., semiconductor layers having the maximum band gap of the superlattice structure.
In U.S. Pat. No. 5,157,473 to Okzaki, an APD and a method for making such is disclosed, including epitaxially growing a window layer of n InP on an avalanche multiplication layer of n.sup.+ InP. The window layer is selectively removed to expose the avalanche multiplication layer, thereby providing a recessed portion. A p-type impurity is then selectively introduced into the window layer, forming a guard ring therein. A second implanting of p-type impurity is then achieved, forming a P-N junction and a guard ring having a gradient dopant concentration.
U.S. Pat. No. 5,075,750 to Kagawa discloses inter alia, an APD having improved RF characteristics. The APD includes a multi-layered structure with an InP layer, multiplication layer, a light-absorbing layer, a light-reflecting layer, and an electrode. The multiplication layer is arranged on one of the InP layers and comprises a superlattice in which In.sub.0.52 Al.sub.0.48 As layers and In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y layers to be lattice-matched with the In.sub.0.52A10.48 As layers are alternatively stacked, thereby improving the gain-bandwidth.
Canadian Patent No. 1,298,640 discloses a SAM-APD and a method for forming such in which a first and a second charge sheet are used to obtain independent control of the electric fields at the central and peripheral portions of the multiplication region and absorption region. The first charge sheet is located between the absorption region and a central portion of a P-N junction of the multiplication layer. The second charge sheet is located between the absorption region and the edges of the P-N junction. The second charge sheet has a lower dopant concentration than the first charge sheet.
The limitations of diffusion techniques for manufacturing APDs are manifest. The precision of diffusion is limited. For example, to achieve a multiplication gain-bandwidth of 100 GHz, a uniformly doped multiplication layer must have a thickness of about 0.7 .mu.m with a required precision of about 4.+-.0.02 .mu.m. It is very difficult to achieve this degree of precision with diffusion technology. For similar reasons, it is difficult to accurately control the doping and position of the guard ring. This often leads to low fabrication yields and increased costs in the production of APDs. Low fabrication yield is a significant drawback in manufacturing an APD with high gain-bandwidth product.
What is needed is the precise control of the doping and thickness of the multiplication layer, the guard ring and an adjacent p.sup.+ -layer, of a mesa structure APD, while avoiding a very high electric field at the surface of the p.sup.+ -layer, as well as an edge breakdown at the interface of the p.sup.+ -layer and the multiplication layer.